1. Field of the Invention
This invention relates in general to speaker enclosures or systems and has specific reference to a unidirectional speaker enclosure or system.
2. Description of the Prior Art
It is known that a speaker system comprises as a rule a support and at least one transducer, notably a loudspeaker, secured to the support and having the front face of its membrane directed towards an aperture of the support.
It is also known that, due to its very conception, a speaker system generates interferences resulting from the fact that the loudspeaker membrane, when a signal is fed to the speaker input, produces concomitant and phase-opposed movements of the air on the one hand forwards and on the other hand backwards of the membrane. The backward wave is equal and in phase opposite with respect to the forward wave, and the addition of these two waves leads to a tone cancellation which of course is not satisfactory.
To solve this problem, two main methods have been used up to now. The first method is directed to deaden the backward wave in order to reduce the negative and detrimental effect produced thereby on the front or forward wave. The other method aims at transforming or "handling" the backward wave in such a manner that it will reinforce, or at least not interfere with, the forward wave.
One way of implementing the first method consists in constructing a so-called "infinite" speaker enclosure in which the support structure consists essentially of a rigid cabinet or box provided with a single front aperture registering with the front face of the speaker membrane, the box being otherwise tightly closed. However, in actual practice this solution is not fully satisfactory. In fact, any membrane movement is attended by a variation in the pressure of the air contained in the box, thus urging the membrane in the direction opposite its normal movement, so that the membrane undergoes a certain damping effect. Now the greater the amplitude of the membrane movement, the higher this damping effect, this greater amplitude corresponding of course to the low-frequency range of operation of the speaker.
This first method is illustrated notably in the French Pat. Nos. 1,587,761 and 2,260,916. In the first French Pat. No. 1,587,761 the speaker system is not unidirectional. In the other French Pat. No. 2,260,916 the speaker system is unidirectional and the cabinet is divided into two compartments interconnected by a valve in order to attenuate the above-mentioned damping effect, however without producing fully satisfactory results.
The second method may be implemented in various ways. A first form of embodiment comprises an enclosure of which the support consists of a flat member having its free edge spaced from the loudspeaker by a distance corresponding substantially to one-half of the wavelength of the sensitive frequency, so that the back wave attaining the front portion of the enclosure is delayed in relation to the front wave to an extent corresponding to one wavelength. In this specific arrangement, it is clear that the enclosure radiates the sound in all directions.
Another form of embodiment of this second method is illustrated by the so-called bass-reflex enclosure in which the backward wave is channelled for on the one hand imparting a certain delay thereto in relation to the forward wave and on the other hand directing the backward wave in a specific direction with respect to the forward wave. This principle is applied in many known constructions which differ notably according to the design of the wave channelling means. Thus, for example, reference may be made in this respect to the U.S. Pat. Nos. 4,033,431, 3,978,941, 3,993,162 and 4,064,966. However, these systems are attended by many inconveniences; thus, notably, the wave channelling means are far from having a complete efficiency. They are generally very elaborate and therefore expensive, cumbersome and heavy. The speaker enclosure proper is also expensive, cumbersome, heavy, and in many cases has a complex configuration ill suited for the use normally contemplated for a loudspeaker enclosure or system. Moreover, the direction or directions and sense of the backward wave radiation do not correspond necessarily to the direction and sense of the forward wave, so that the speaker system is in most instances non-directive or still multi-directional.
For certain specific applications, notably the sound installation in restrictive volumes and the use of microphones in rooms of reduced volumes, the use of unidirectional speaker enclosures, i.e. having a single direction and a single sense of sound radiation, in lieu of multi-directional or non-directional speaker enclosures, is required.
Now, as already explained in the foregoing, speaker systems of the so-called infinite type and also of the flat type are not capable of producing this unidirectional sound radiation. Speaker enclosures of the bass reflex type are also either ill-suited, since the backward wave is transmitted in a direction or a sense other than those of the forward wave, or unsatisfactory since the backward wave channelling means have the above-mentioned inconveniences.